1. Technical Field
The present invention generally relates to thin films containing organometallic or inorganic compounds with high extreme ultraviolet (EUV) optical density (OD) and high mass densities as high resolution, low line edge roughness (LER) EUV photoresists.
2. Background Information
A cycle of a typical silicon lithography procedure begins by applying a layer of photoresist—a material that undergoes a chemical transformation when exposed to actinic radiation (generally but not necessarily visible light, ultraviolet light, electron beam, or ion beam)—to the top of the substrate and drying the photoresist material in place, a step often referred to as “soft baking” of the photoresist, since typically this step is intended to eliminate residual solvents. A transparent plate, called a photomask or shadowmask, which has areas that are opaque to the radiation to be used as well as areas that are transparent to the radiation, is placed between a radiation source and the layer of photoresist. Those portions of the photoresist layer not covered by the opaque areas of the photomask are then exposed to radiation from the radiation source. Exposure is followed by development. In some cases, exposure is followed by a post-exposure bake (PEB), which precedes the development. Development is a process in which the entire photoresist layer is chemically treated. During development, the exposed and unexposed areas of photoresist have different solubility properties, so that one set of areas is removed and the other remains on the substrate. After development, those areas of the top layer of the substrate which are uncovered as a result of the development step are removed using an etch process. When a “positive” photoresist is used, the opaque areas of the photomask correspond to the areas where photoresist will remain upon developing (and hence where the topmost layer of the substrate, such as a layer of conducting metal, will remain at the end of the cycle). “Negative” photoresists result in the opposite—any area that is exposed to radiation will remain after developing, and the masked areas that are not exposed to radiation will be removed upon developing.
The need to make circuits physically smaller has steadily progressed over time, necessitating inter alia the use of light of increasingly shorter wavelengths to enable the formation of these smaller circuits. At present, it is desired to use light in the extreme ultraviolet (EUV) range (13.5 nm or shorter) for photolithography of circuits having line widths of 32-20 nm. This in turn has necessitated changes in the materials used as photoresists, since in order to be useful as a photoresist, the material should absorb some of the light at the wavelength used. If too much light is absorbed, then the light may not reach the bottom of the resists and the sides walls of the printed feature will be slanted rather than vertical. For example, phenolic materials which are commonly used for photolithography using light of wavelength 248 nm wavelength are generally not suitable for use as photoresists for light of 193 nm, since these phenolic materials tend to absorb too much 193 nm light. Conversely, if too little light is absorbed, then the exposure times will need to be long, resulting in poor manufacturing through-put, and higher manufacturing costs. X-ray lithography (˜1 nm wavelength of light) has very long exposure times because very little of the x-rays are absorbed. [R. Brainard, G. Barclay, E. Anderson, L. Ocola, “Resists for next generation lithography”, Microelectron. Eng. 61-62, 707-715 (2002).]
Extreme ultraviolet (EUV) photoresists are being driven to provide better resolution and sensitivity. In order to do so, the photoresists have to become thinner and thinner, but as they become thinner, it becomes harder to stop photons.
A need exists for resists with extremely high resolution imaging in EUV that will be capable of meeting the needs of the 10-nm node. In particular, a need exists for extreme ultraviolet (EUV) photoresists with high resolution imaging and low line edge roughness (LER).